In a wireless local area network (WLAN), there are generally two network mechanisms that allow wireless devices to get channel access: distributed and centrally-coordinated. In the distributed mechanism, each mobile wireless device (STA) may try to transmit when it thinks the channel is going to be free, but sometimes multiple STAs transmit at the same time and thereby interfere with each other, causing a need for retries. In the centrally-coordinated mechanism, a coordinating device, such as an access point (AP), may poll each STA in turn, giving the polled STA exclusive use of the channel for a while. This may prevent collisions and retries by allowing only one STA to transmit at a time, but some STAs may experience communication delays because they have to wait while the AP is polling other STAs that have nothing to transmit.
Rather than waiting for its turn to be polled, a STA with data to transmit may send a short request to the central coordinator that the STA be polled for one or more longer-duration transmissions. Alternately, the central coordinator may send out unsolicited polls to one or more STAs. Which of the methods (contention for the channel, unsolicited polls, or solicitations to be polled) provides the best overall channel throughput may depend on various things such as channel conditions, which may change dynamically with time. Conventional networks may send out unsolicited polls on a fixed schedule, a technique that responsive to changing channel conditions.